JERUSALEM — Avigdor Lieberman, Israel’s foreign minister, stunned the political establishment Monday by announcing he would not join the next government, leaving Prime Minister Benjamin Netanyahu with two fraught options: a conservative coalition whose razor-thin majority would be inherently unstable, or a unity government riven over how to deal with the state’s critical challenges.

With two days until the legal deadline for Mr. Netanyahu to announce a new coalition or hand the chance to form a government over to his center-left rival, a spokesman for the prime minister and his Likud Party declined to respond to Mr. Lieberman’s move. But political analysts said it places Mr. Netanyahu, six weeks after winning a strong mandate for a fourth term in the divisive March 17 election, in a weak and sticky situation.

In Israel’s fractious parliamentary system, politicians cobble together governing coalitions through weeks of back-room deal-making. Mr. Netanyahu, whose Likud Party won 30 of Parliament’s 120 seats, was expected to form a 67-member coalition made up of six right-leaning and religious parties; without Mr. Lieberman, that ideologically coherent group would have 61.

“It puts him in a corner in which it will be much more difficult to maneuver,” said Maoz Rosenthal, a professor of government and an expert on coalition-building at the Interdisciplinary Center in Herzliya. “Once Lieberman is out of the bubble, it means Netanyahu is at the hand of small parties. That coalition is bound for trouble, especially when you don’t have a balancing factor somewhere in the middle. It puts him in a really tough spot, no doubt about it.”

Mr. Lieberman, a polarizing figure who has called for executing terrorists and transferring Arab areas of Israel to Palestinian control, said Monday that he was quitting his post because the coalition under construction was “opportunist” and not “nationalist.”

He said his Yisrael Beiteinu party, which won six seats in the election, chose principles over ministerial portfolios. He said the new government under consideration would reverse recent moves to ease conversion to Judaism and draft more ultra-Orthodox men into the military. But, he said, it would not pass the so-called nationality bill emphasizing Israel’s Jewish character or uproot the Islamist movement Hamas from the Gaza Strip. “As far as we are concerned, a promise is a promise, not just a slogan, it is a way of life,” Mr. Lieberman wrote on his Facebook page. “We will continue to stick to our beliefs and fight for them from within the opposition.”

Though many political commentators suspect Mr. Netanyahu is now much more likely to join forces with the center-left Zionist Union, which had tried to unseat him, that group’s leader, Isaac Herzog, on Monday reiterated his plan to lead “a strong and fighting opposition.”

“There was never a chance for a national-unity government, and all the background noise, spins and talks were nothing,” Mr. Herzog said on Army Radio, comments that themselves were quickly dismissed as yet more spin as the coalition-forming clock ticks down. “Now everyone knows that this is yet another government of failure that will be weaker, more susceptible to extortionate demands, more than it’s ever been, and from its very first day.”

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Israeli Prime Minister Benjamin Netanyahu, center, has just two days until the legal deadline to announce a new coalition.CreditRonen Zvulun/Reuters

But Moshe Kahlon, whose new Kulanu party has already signed a coalition agreement, said on Monday that a government of 61 “is not good as far as the many challenges ahead” and that he was “confident” Mr. Netanyahu “will call on additional factions or will convince Lieberman to come back.” And Aryeh Deri, whose ultra-Orthodox Shas faction joined the coalition hours after Mr. Lieberman dropped out, according to Israeli news sites, called on Mr. Herzog to come inside as well.

A government that includes the Zionist Union — likely with Mr. Herzog as foreign minister — could help repair the tattered relations between Mr. Netanyahu and the White House and fend off European efforts to punish Israel for its failure to make peace with the Palestinians. While Mr. Netanyahu called the early elections in hopes of replacing a coalition fractured over the Palestinian issue, among others, with one that hewed more to his hawkish views, he has in the past preferred having a left-leaning presence in his governments.

“He can’t really be at 61 seats, it’s almost impossible to govern that way — any one party can bring it down, any two members of Parliament can,” said Mitchell Barak, a Jerusalem political consultant who once worked for Mr. Netanyahu. “The best-case scenario is to bring in the Zionist Union, which I think he wants to do because not only can he govern better, but it helps him face the international community when he has the center and the center-left when it comes to peace issues.”

If Mr. Herzog indeed does an about-face and teams up with Mr. Netanyahu after campaigning vigorously against him, there might be some defections among the Zionist Union’s 24 Parliament members. But there might also be a historic opportunity for the leader of the Arab contingent in parliament: Ayman Odeh, who heads the new joint list of Arab parties that won 13 seats, would have the chance to be opposition leader with access to security briefings, meetings with visiting foreign leaders, and formalized rights and privileges.

The likeliest scenario, several analysts said, is that Mr. Netanyahu will by Wednesday’s midnight deadline form the narrow 61-member coalition, and later lure in either Mr. Herzog or Mr. Lieberman.

Besides Shas and Kulanu, a party that broke away from the Likud and is focused on economic issues, United Torah Judaism, another Orthodox party, has already joined the coalition. To get to the barest majority of 61, Mr. Netanyahu still needs the Jewish Home, and Mr. Lieberman’s withdrawal gives the party more leverage in already-tense negotiations over portfolios and policy positions.

Even Mr. Lieberman could be playing chicken to get more of what he has been bargaining for in coalition talks. “It’s not the final word,” warned Shmuel Sandler, a political scientist at Bar-Ilan University. “He likes to surprise, be unpredictable, that’s his fun.” Mr. Lieberman, an immigrant from the former Soviet Union who at the end of 2013 was acquitted of corruption charges that dogged him for more than a decade, has alternated between ally and antagonist of Mr. Netanyahu. Their parties ran together in the 2013 election, but this time Mr. Lieberman was one of the prime minister’s fiercest critics.

With Yisrael Beiteinu having shrunk to six seats from 15 in 2009, when Mr. Lieberman first became foreign minister, his political future was already in doubt. Casting aspersions from the opposition — particularly when Mr. Netanyahu will have to preside either over a coalition with the thinnest of majorities or a fractious unity government unpopular with the public — could help him win back hard-line voters and revive his ambitions to one day become prime minister.

“Lieberman is a long-term player,” Professor Rosenthal said. “He sees that Netanyahu is going to be under a huge amount of pressure, and he’s going to cave this way or that, and Lieberman can simply wait for him to fall.”

Correction:

An article on Tuesday about an announcement by Avigdor Lieberman, Israel’s foreign minister, that he was resigning and would not join the next government referred imprecisely to his position on Israeli-Palestinian boundaries. He has called for Arab areas of Israel to be transferred to Palestinian control; he has not called for the “transferring” of individual Arab citizens to Palestinian territory.

Gabby Sobelman contributed research.

A version of this article appears in print on , on Page A7 of the New York edition with the headline: Netanyahu Ally Declines to Join Government, Putting Premier in Bind. Order Reprints | Today’s Paper | Subscribe